The subject invention relates generally to alignment of printheads and more specifically to a method for aligning one or more printheads, such as color and black printheads, which permits accurate subjective alignment determination.
Various technologies are well known for effecting printing on media. For example, laser printers, heat sublimation printers, inkjet printers, thermal printers, and the like, are well known. Color printers often have a plurality of print heads. For example, a typical color inkjet printer has four inkjet printheads, one that utilizes black ink, and three that utilize colored inks, such as magenta, cyan and yellow. The colors from the three color printheads are mixed to obtain any desired color.
A typical raster or matrix type printhead is capable of printing a plurality of rows of dots in a single scan across the print media. Each printhead typically includes a plurality of printing elements (e.g., ink jet nozzles) displaced relative to each other which allow printing of a plurality of rows of dots. After one scan, the media can be advanced by the number of rows that the each printhead is capable of printing in one scan.
A consideration with multiple printhead printers is print quality degradation, such as poor color registration, as a result of printhead mechanical alignment (e.g., the uncertainty of printhead to printhead positioning, and drop velocity differences between printheads). Mechanical tolerances of the printhead to print media spacing also contribute to poor registration. Factory compensation for each printer manufactured and/or tight manufacturing tolerance control would address some of the factors contributing to printhead misalignment, but would be extremely difficult and expensive. Moreover, manufacturing tolerance control might not be able to address the alignment effects of aging and temperature. Alignment of printheads is also a concern after manufacturing by customers and other end users of these printers. Accordingly, methods of xe2x80x9csoft alignmentxe2x80x9d have been developed which permit printhead alignment selection using software. In particular, the timing of ink ejection and ejector selection is adjusted to create proper alignment of printed dots on the print media. The term xe2x80x9calignmentxe2x80x9d as used herein refers to correction of offsets due to mechanical tolerances, ink ejection direction and velocity, and other factors contributing to poor registration between data printed by printheads. One known technique of alignment involves printing to test pattern of vertical and horizontal lines corresponding to various incremental offsets of the printheads. The user then selects portions of the test pattern in which the horizontal and vertical lines most closely represent a straight unbroken line. For example, U.S. Pat. No. 4,878,063, which is herein incorporated by reference, discloses such a method.
However, the determination of which lines on the test pattern most closely represent a straight unbroken line is highly subjective and often difficult for a user to make. Of course any errors in the determination result in poor offset data being used for soft alignment and thus poor registration of colors.
A first aspect of the invention is a method of aligning one or more print heads in a print head unit. The method comprises printing a first pattern of first symbols on a print medium and printing a second pattern of second symbols with one or more print heads on the print medium in a manner to superpose the second pattern on the first pattern and so that at least some of the first symbols obscure a corresponding one of the second symbols. At least some of the second symbols are offset from a regular pattern and the offset distances are recorded. A user designates at least one of the second symbols that is obscured by a corresponding one of the first symbols and an offset distance corresponding to the designated second symbol is used as an offset parameter for aligning one of the one or more print heads.
A second aspect of the invention is a method of creating a test print for use in aligning one or more print heads in a print head unit. The method comprises printing a first pattern of first symbols on a print medium and printing a second pattern of second symbols on the print medium in a manner to superpose the second pattern on the first pattern so that at least some of the first symbols obscure a corresponding one of the second symbols. At least some of the second symbols are offset from a regular pattern.
A third aspect of the invention is a system for aligning one or more print heads in a print head unit including one or more print heads and a controller. The one or more print heads print a first pattern of first symbols on a print medium and print a second pattern of second symbols on the print medium in a manner to superpose the second pattern on the first pattern and so that at least some of the first symbols obscure at least portions of a corresponding one of the second symbols. The controller records an offset distance of the second pattern of symbols from a regular pattern, designates at least one of the second symbols that is most completely obscured by a corresponding one of the first symbols, and uses the offset distance corresponding to the designated second symbol as an offset parameter for aligning one of the one or more print heads.